Female reproductive output in exploited Nile monitor lizard (Varanus niloticus L.) populations in Sahelian Africa

Abstract

Sexual maturation and ontogenetic development of gametes were studied in a sample of 481 female Nile monitor lizards (Varanus niloticus L.) (18.8-71.5 cm snout-vent length (SVL)) from the Lake Chad region in Chad and Nigeria and the central delta of the Niger River in Mali. The timing of egg maturation suggests that only one large clutch can be produced in 12 months, and that egg laying occurs between late November and early January. Each year, about 45-50% of mature females, on average, lay eggs, but local sampling shows that this proportion varies greatly. Females reach sexual maturity at 36 ± 2 cm SVL and 24 months of age. Clutch size (CS) increases from 12-13 to 53-60 eggs (average values for specimens of 36 cm and 71.5 cm SVL, respectively) with increase in SVL, according to the power function CS = 0.0049 SVL2.206 (specimens from Mali) or CS = 0.0052 SVL2.163 (specimens from the Lake Chad region). Conversely, the relative mass of the clutches (RCM) and their relative energy content (RWC) decrease during growth; hence, small (young) individuals are subjected to a much higher reproductive effort than large ones. Compared with that of most other large varanids, egg mass in the Nile monitor (38.8 g) is small, but reproductive output (CS) and effort (RCM, RWC) appear to be very high. Moreover, the specimens from Mali, where the species is heavily exploited, have a higher reproductive output and shorter longevity than those from the Lake Chad region. These data are discussed with reference to the ecology and local level of exploitation of the Nile monitor.